Transport refrigeration systems have been supplying conditioned air for fresh loads for many years, and the manufacturers of such systems have all been striving to accomplish the same goals. The goals are to maintain the temperature of the served space as close as possible to a selected set point temperature, with a uniformity which prevents damage to the load, and to maintain a high relative humidity in the served space.
Commercially available transport refrigeration systems usually maintain a desired set point temperature by continuously cycling between cooling and heating modes. Some systems include a null mode between the heating and cooling modes, during which a prime mover for the refrigerant compressor is shut down. Trailer refrigeration units commonly utilize a Diesel engine as a prime mover for operating a refrigerant compressor. Some trailer systems additionally have an electric stand-by motor for operating the compressor when the trailer is parked near a source of electric potential. Refrigerated containers or "reefers" commonly employ only an electric motor, with power being supplied from a Diesel engine driven electrical generator when the container is on a truck, and otherwise from electrical mains when the container is on board a ship or at a terminal.
A spring biased throttling valve is commonly placed in the suction line of the refrigerant compressor to protect the compressor and prime mover against overload when operating with high refrigerant pressure. The throttling valve reduces the thermal capacity of the system at times when an overload is not involved, however, and it is particularly inefficient in dual prime mover systems which may drive the compressor directly from a Diesel engine, or directly from an electric motor. The electric motor does not have as much power as the Diesel engine, and the spring of the throttling valve must be set to protect the electric motor, resulting in reduced thermal capacity when operating with the Diesel engine.
Diesel engine driven compressors of the prior art which operate continuously, as opposed to shutting down the Diesel engine when set point is reached, cycle continuously between cooling and heating modes to hold the desired set point temperature. The Diesel engine has ample driving power and is usually operated at high speed, such as about 2200 RPM, only during pull down, to rapidly bring the temperature of the served space to a predetermined temperature above set point, at which point the engine speed is reduced to a lower speed, such as 1400 RPM. The Diesel engine will then usually operate at the lower speed during the ensuing cycling back and forth between cooling and heating modes, reverting to high speed to hold set point only during cooling and heating modes associated with very high and very low ambient temperatures, respectively.
Thermal capacity, even with prior art systems which include compressor unloading, is such that with continuously operated compressors, the temperature difference between the entrance and exit air from the evaporator is relatively high, removing considerable moisture from the conditioned air via condensation on the evaporator coils. The significant thermal capacity of the prior art systems about set point also causes temperature swings which require maintaining the average temperature of the served space higher than optimum to prevent freezing of the load. Some container systems have utilized a controlled suction line modulation valve to gradually reduce thermal capacity during a cooling mode, to approach set point and then turn the electric motor off and enter a null mode as set point is reached.
The optimum average load temperature for fresh produce and cut flowers is as close to freezing as possible without damaging the load. Temperature swings which approach freezing and then are driven up during an ensuing heating cycle, result in an average load temperature several degrees above freezing. If the average load temperature could be maintained near freezing without damaging the load, and the relative humidity of the served space can be maintained at a high level, the shelf life of the produce or fresh cut flowers may be increased by as much as 100%, resulting in tremendous savings for the merchandiser